1. Field of the Invention
The present invention relates to a vapor delivery system, and more particularly, to a system for delivering a controlled and stable vapor flow of vaporized liquid and solid source materials for use in chemical vapor deposition (CVD) and ion implantation processes.
2. Description of the Related Art
Chemical vapor deposition (CVD) has been extensively used for preparation of films and coatings in semiconductor wafer processing. CVD is a favored deposition process in many respects, for example, because of its ability to provide highly conformal and high quality films, at relatively fast processing times. Further, CVD is beneficial in coating substrates of irregular shapes including the provision of highly conformal films even with respect to deep contacts and other openings.
In general, CVD techniques involve the delivery of gaseous reactants to the surface of a substrate where chemical reactions take place under temperature and pressure conditions that are favorable to the thermodynamics of the desired reaction. The type and composition of the layers that can be formed using CVD is limited by the ability to deliver the reactants or reactant precursors to the surface of the substrate. Various liquid precursors are successfully used in CVD applications by delivering the precursor vapor in carrier gas. For example, liquid precursor can be delivered by bubbling carrier gas at a controlled rate through a container of the vaporized precursor. The carrier gas is saturated with vaporized liquid precursor and is then transported to the CVD reaction chamber.
Analogous attempts have been made to deliver solid reactants to a CVD reaction chamber, but with much less success. The delivery of solid precursors in CVD processing is carried out using the sublimator/bubbler method in which the precursor is usually placed in a bubbler reservoir which is then heated to a temperature at which the precursor has a reasonable vapor pressure. The precursor vapor then is transported into the CVD reactor with a carrier gas such as hydrogen, helium, argon, or nitrogen. However, this procedure has proven to be problematic because of the inability to deliver, at a controlled rate, a reproducible flow of vaporized solid precursor to the process chamber.
Similar problems are inherent in conventional ion implantation systems wherein a dopant element is ionized and then subsequently accelerated to form an ion beam directed at a workpiece surface for implantation. In ion implantation processes, there are a number of frequently used dopants, such as arsenic, phosphorus, boron, antimony, indium, etc. Ideally, the dopant is delivered as a gas. However, a solid compound may be delivered via a heated vaporization vessel, which usually does not have any flow control devices. As such, the vapor flow rate often varies with time, due to temperature and changes in the solid surface area.
Solid ion source material is greatly preferred for safety reasons, however, solid semiconductor dopants have presented serious technical and operating problems. For instance, the changing surface area of the bulk solid precursor produces a continuously changing rate of vaporization, particularly for thermally sensitive compounds. This ever-changing rate of vaporization results in a continuously changing and non-reproducible flow of vaporized solid precursor for use in the process chamber. Thus, at present, processes using such vaporized solid precursors cannot be controlled adequately and effectively.
In typical systems used for CVD and ion implantation, it is necessary to precisely meter the output mass flow rate of vaporized constituents. Regulation of vapor mass flow is accomplished in the prior art in various ways. For example, the temperature and pressure of the material in the vaporizer are closely regulated to maintain a constant flow of vapor-phase material. Also, the flow rate of a carrier gas through the vaporizer is controlled in an attempt to completely saturate the carrier gas with vaporized source material. However, these methods still present problems because of the variations in the amount of source material being vaporized within the vessel before combining with the carrier gas.
Accordingly, there is need in the art for a vaporizer system that efficiently vaporizes and delivers vaporized solid or liquid source materials at a highly controllable and reproducible flow rate.